China’s Vice President Xi Jinping’s visit to Washington next week comes at a time when tensions between Iran and the international community are escalating over Iran’s nuclear program. Xi, the next likely president of China, has a crowded official agenda on a broad range of issues with the Obama administration and Congress, including trade and global security concerns like Iran’s nuclear program.

Over the past three years, the Obama administration took important steps to make China a part of a new international consensus demanding accountability from Iran over its nuclear program. To accomplish this goal, the United States engaged in an assertive global diplomatic campaign to bring pressure to bear on Iran over its failure to come clean over its nuclear intentions. Taking the Iranian nuclear threat seriously, the Obama administration provided unprecedented defense cooperation with regional allies, assembled an international coalition that today leaves Iran more isolated than ever, and implemented targeted economic sanctions to bring Iran back to the negotiating table.

Engagement with China on Iran is a major component of the Obama administration’s diplomacy, for good reason. China’s economic growth is directly fuelled in large part by imported Iranian oil and gas, since Iran is China’s third-largest crude oil supplier after Saudi Arabia and Angola. Any disruption to oil flowing out of the Persian Gulf in a conflict with Iran would send up oil prices.

But is China listening to U.S. diplomats? On the face of it, it would seem not. In the name of its own energy security interests, China deals with pariah countries such as Sudan and Iran, which puts it at odds with the global consensus. Also, as we saw in last weekend’s vote at the United Nations on Syria, China remains willing to align itself with Russia in the United Nations veto that blocked widely popular international efforts to further isolate the brutal Assad regime.

So, in public at least, China does not seem to be undertaking much of a shift on Iran. Indeed, just last month U.S. Treasury Secretary Timothy Geithner visited China in part to press for China’s support in isolating Iran because of its nuclear program. China offered a mixed public response, with Chinese foreign ministry spokesmen insisting the dispute over Iran’s nuclear program “cannot be resolved by sanctions alone.” This was hardly the unequivocal support the United States had presumably hoped for.

But actions speak louder than words. In fact, China has made important changes in its policy on Iran in recent years—in large part due to the Obama administration’s assertive diplomacy on Iran. The most recent news—that China decided to continue to cut its oil imports from Iran next month compared to March 2011—is an important step. And in 2010, China supported United Nations Security Council Resolution 1929, the most extensive package of sanctions Iran has ever faced. China did so despite a last-minute diplomatic effort by Turkey, Brazil, and Iran to avert its passage.

China is also apparently partially complying with the Comprehensive Iran Sanctions, Accountability and Divestment, enacted by the United States in 2010. Chinese firms have scaled back their activities in Iran in response to instructions from both Chinese leadership and their own business calculations. Chinese national energy companies such as the China National Petroleum Company, China National Offshore Oil Corporation, and the Sinopec Group have all slowed or halted work on multibillion dollar Iranian energy development projects such as the North and South Pars gas field and the Yadavaran oil field. Although these are positive developments, the challenge remains in getting multinational corporations around the world, including Chinese companies, to comply with existing sanctions on Iran.

China has also taken steps to diversify its oil supply so that it is not so reliant on Iranian crude. China has nearly doubled imports from Saudi Arabia and dramatically increased imports from other suppliers such as Iraq. China is also planning to diversify its energy supplies away from the troubled Persian Gulf, as evidenced by Sinopec’s plan to build a $10 billion, 400,000-barrel-a-day refinery on the Saudi Red Sea Coast. And during a recent visit by Premier Wen Jiabao to the Gulf region, China National Offshore Oil Corp. unveiled plans to build a refinery at Taizhou, on the coast of China’s Jiangsu province in a joint venture with Qatar Petroleum International and Royal Dutch Shell Group.

These shifts have developed over the past couple of years. They indicate that the Obama administration’s patient diplomacy with China over Iran is having an impact. While it remains extremely unlikely China will adopt a full boycott against the regime in Tehran, China’s diplomatic messages to Iran are also becoming more emphatic, reinforcing the international message that Iran must be held accountable for its uncertain nuclear intentions.

China’s step forward is evident in its leaders publicly voicing their concerns about the Iranian nuclear program, a sharp contrast to China’s past position. Most recently, Premier Wen stated that China “adamantly opposes Iran developing and possessing nuclear weapons.” His recent visit to the Persian Gulf region in January marked the first in 21 years for a Chinese premier to visit Saudi Arabia. And his visit to the United Arab Emirates and Qatar were the first by a Chinese premier. Even more striking is that Wen’s trip did not include a visit to Iran.

Encouraging fuller compliance with U.N. sanctions by China during Vice President Xi’s upcoming visit to Washington will be a critical U.S.-China summit item in the brief Washington visit that is already packed with important bilateral issues including trade, Asia-Pacific regional issues, as well as Xi’s introduction to summit politics as China’s successor to current president, Hu Jintao. But don’t expect unbridled support for U.S.-led sanctions against Iran from Xi while he is in Washington. Instead, study the nuanced diplomatic language he uses when discussing Iran—and then look for further words and action by China in the coming months akin to steps it has already taken in recent years to get behind sanctions against Iran.

Specifically, the United States should encourage China to use its diplomatic leverage with Iran to de-escalate tensions in the region by halting Iranian threats and pushing for Iran to make a good-faith effort at negotiations with the so called P5+1 group of nations (which includes the United States, China, Russia, Great Britain, France, and Germany) which are trying to enforce global nonproliferation rules in Iran. Escalating tensions in the Persian Gulf are manifestly not in Chinese economic, political, or national security interests. The United States should make it clear to Xi that we recognize what’s at stake for China in the Persian Gulf and Iran to ensure Beijing exercises its influence with Tehran in a manner that contributes toward reducing those tensions.

The specter of military confrontation with Iran should provide added impetus for Chinese diplomacy to bring down tensions in the region and ensure Iran is held accountable for its opaque nuclear ambitions. The United States should seize upon the danger of the moment to encourage China play an active constructive role in reducing tensions between Iran and the rest of the world.

Rudy deLeon is Senior Vice President for National Security and International Policy. Brian Katulis is a Senior Fellow with the National Security team at the Center. Peter Juul and Ali Fisher are Policy Analysts with the National Security team.